U.S. patent application number 10/458109 was filed with the patent office on 2004-05-27 for fiber grating bond joint health monitoring system.
Invention is credited to Schulz, Whitten Lee, Udd, Eric.
Application Number | 20040099801 10/458109 |
Document ID | / |
Family ID | 26869056 |
Filed Date | 2004-05-27 |
United States Patent
Application |
20040099801 |
Kind Code |
A1 |
Schulz, Whitten Lee ; et
al. |
May 27, 2004 |
FIBER GRATING BOND JOINT HEALTH MONITORING SYSTEM
Abstract
Fiber grating environmental measurement systems are comprised of
sensors that are configured to respond to changes in moisture or
chemical content of the surrounding medium through the action of
coatings and plates inducing strain that is measured. These sensors
can also be used to monitor the interior of bonds for degradation
due to aging, cracking, or chemical attack. Means to multiplex
these sensors at high speed and with high sensitivity can be
accomplished by using spectral filters placed to correspond to each
fiber grating environmental sensor. By forming networks of spectral
elements and using wavelength division multiplexing arrays of fiber
grating sensors may be processed in a single fiber line allowing
distributed high sensitivity, high bandwidth fiber optic grating
environmental sensor systems to be realized.
Inventors: |
Schulz, Whitten Lee;
(Fairview, OR) ; Udd, Eric; (Fairview,
OR) |
Correspondence
Address: |
Eric Udd
Blue Road Research
P.O. Box 667
Fairview
OR
97024
US
|
Family ID: |
26869056 |
Appl. No.: |
10/458109 |
Filed: |
June 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10458109 |
Jun 10, 2003 |
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09746037 |
Dec 22, 2000 |
|
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6600149 |
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60173359 |
Dec 27, 1999 |
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Current U.S.
Class: |
250/227.14 |
Current CPC
Class: |
G02B 6/022 20130101;
G01M 3/165 20130101; G01N 2021/7723 20130101; G01M 3/38 20130101;
G01N 21/774 20130101 |
Class at
Publication: |
250/227.14 |
International
Class: |
G01J 001/04; G01J
001/42; G01J 005/08 |
Claims
What is claimed is:
1. An environmental sensor to measure strain fields interior to a
bond joint including: (a) an optical fiber grating written onto
birefringent optical fiber, and (b) the fiber placed in a bond
joint.
2. An environmental sensor as recited in claim 1 including: (a)
said optical fiber grating axes oriented at 45 degrees to plane of
the joint.
3. An environmental sensor as recited in claim 1 including: (a)
said optical fiber grating near the edge of the bond.
4. An environmental sensor as recited in claim 1 including: (a)
said optical fiber grating being outside the edge of the bond.
Description
[0001] This application is a divisional of patent Ser. No. ______,
granted ______. This application claims the benefit of U.S.
Provisional Application No. 60/173,359 by Whitten L. Schulz, John
Seim and Eric Udd, entitled, "Fiber Grating Environmental Sensing
System" which was filed on Dec. 27, 1999. This invention was made
with Government support from contract DE-FG03-99ER82753 awarded by
DOE and by contracts N68335-98-C-0122 and N68335-99-C-0242 awarded
by NAVAIR. The government has certain rights to this invention.
REFERENCE TO RELATED PATENTS
[0002] This disclosure describes means to enhance the speed and
sensitivity of fiber grating sensors systems to measure
environmental effects and means to multiplex these sensors while
retaining high speed characteristics. The background of these types
of fiber grating sensors may be found in U.S. Pat. Nos. 5,380,995,
5,402,231, 5,592,965, 5,841,131 and 6,144,026. The teachings in
those patents are incorporated into this disclosure by reference as
though fully set forth below.
BACKGROUND OF THE INVENTION
[0003] This invention relates generally to fiber optic grating
systems and more particularly, to the measurement of environmental
effects using fiber optic grating sensors. Typical fiber optic
grating sensor systems are described in detail in U.S. Pat. Nos.
5,380,995, 5,402,231, 5,592,965, 5,841,131 and 6,144,026.
[0004] The need for low cost, a high performance fiber optic
grating environmental sensor system that is capable of long term
environmental monitoring, virtually immune to electromagnetic
interference and passive is critical for such applications as
moisture sensing and monitoring of adhesive bonds. Another
advantage of these system is that when they are appropriately
configured the frequency response of the system can be very
high.
[0005] The present invention includes multi-axis fiber grating
sensors that may be used to sense axial strain and temperature, or
axial and transverse strain simultaneously to detect chemical
changes such as moisture by using appropriate transducers or
changes to the structural integrity of coatings such as adhesive
bonds. Means are also described to multiplex these fiber grating
sensors allowing high sensitivity and high speed measurements to be
made.
[0006] In U.S. Pat. Nos. 5,380,995 and 5,397,891 fiber grating
demodulation systems are described that involve single element
fiber gratings and using spectral filters to demodulate fiber
gratings. The present invention includes means to extend the
demodulation system to multiple fiber grating sensors operating at
high speed on a single fiber line. In U.S. Pat. Nos. 5,591,965,
5,627,927 the usage of fiber gratings to detect more than one
dimension of strain is described. These ideas are extended in U.S.
Pat. Nos. 5,869,835, 5,828,059 and 5,841,131 to include fibers with
different geometries that can be used to enhance sensitivity or
simplify alignment procedures for enhanced sensitivity of
multi-parameter fiber sensing. In U.S. patent application Ser. No.
09/176,515, "High Speed Demodulation Systems for Fiber Grating
Sensors", by Eric Udd and Andreas Weisshaar means are described to
process the output from multi-axis fiber grating sensors for
improved sensitivity. All of these patents teaching are background
for the present invention which optimizes the fiber grating sensor
for optimum response to strain changes induced by changes in the
state of its coating or surrounding media to form water/chemical
sensors and monitor the status of adhesive joints through
measurements of strain interior to the bond.
[0007] The present invention consists of an optical fiber whose
axial, transverse and or temperature sensitivity has been optimized
through the construction of the optical fiber or mechanical
mechanisms to enhance sensitivity. High speed demodulation is
provided by wavelength division multiplexing of these fiber grating
sensors using series of fiber grating filters. The spectral filters
are arranged so that each fiber grating sensor has a corresponding
filter to match it, allowing higher speeds and sensitivity than
many current approaches. To sense transverse strain at high speeds
in birefringent optical fiber, the two spectral peaks associated
with the fiber gratings are tracked individually by locking onto
its preferred polarization state.
[0008] Therefore, it is an object of the present invention to
monitor changes in moisture or chemical content of an environment
through measured strain changes.
[0009] Another object of the invention is to provide a means of
monitoring bond lines for degradation.
[0010] Another object of the invention is to provide means to
measure changes in several fiber grating sensors at high speed and
with high sensitivity simultaneously in a single fiber.
[0011] Another object of the invention is to measure transverse
strain as well as axial strain at high speed and with high
sensitivity.
[0012] These and other objects and advantages of the present
invention will become apparent to those skilled in the art after
considering the following detailed specification including the
drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a prior art illustration of a grating written onto
sidehole fiber.
[0014] FIG. 2 is a diagram showing the splitting of a spectral peak
with transverse loading on grating written onto ordinary single
mode fiber.
[0015] FIG. 3 is a diagram showing the separation of spectral peaks
with transverse loading of a grating written onto PM fiber.
[0016] FIG. 4 is an illustration showing the basis of a fiber
grating chemical sensor with a chemically sensitive coating
attached to plates which are constricted and strain the grating as
the coating swells in the presence of the target chemical.
[0017] FIG. 5 is an illustration of a chemical sensor employing m
and n stacks of a chemical sensitive coating to change sensitivity
of sensor
[0018] FIG. 6 is an illustration of a fiber embedded into composite
tow bonded to stiff plates. As the chemically sensitive coating
expands or contracts, the strain state in the fiber grating sensor
changes.
[0019] FIG. 7 is an illustration showing a fiber grating embedded
into composite part. As the affinity coating changes, the strain on
the sensor will change.
[0020] FIG. 8 is an illustration of a fiber grating sensor with a
single v-groove plate to prevent fiber rotation.
[0021] FIG. 9 is an illustration of a fiber grating sensor with a
double v-groove design to eliminate possible rocking of the top
plate.
[0022] FIG. 10 is an illustration showing the use of channels to
prevent the top plate from rocking on fiber.
[0023] FIG. 11 is an illustration showing multiple sensing points
for extended sensing range or higher accuracy through
averaging.
[0024] FIG. 12 is an illustration showing the use of beveled plate
to increase surface area of exposed coating and/or increase wicking
action of target chemical into coating.
[0025] FIG. 13a, FIG. 13b, and FIG. 13c, show different methods to
increase target chemical absorption through the transducer
plates.
[0026] FIG. 14 is an illustration showing how a flexible plate may
be used to account for inconsistent swelling of the chemically
sensitive coating.
[0027] FIG. 15 is an illustration showing the placement of the
coating directly on the fiber.
[0028] FIG. 16 is an illustration showing the placement layers of
chemically reactive composite tow over the fiber which may load the
fiber in transverse strain when exposed to the target chemical. For
example, some composite tows may swell in the presence of moisture.
The void may be used to ensure a clean transverse load.
[0029] FIG. 17 is an illustration showing the wing of an aircraft
where transverse fiber grating strain sensors are used to monitor
the adhesive joints.
[0030] FIG. 18 is an illustration of a transverse fiber grating
strain sensor embedded, directly into the adhesive of a bond to
monitor the health of the bond.
[0031] FIG. 19 is an illustration of three different embedding
locations of transverse strain sensors into an adhesive joint.
[0032] FIG. 20a is a diagram showing uniform loading with clean
spectral peaks and FIG. 20b shows non-uniform loading with more
complex spectral profiles of gratings written onto polarization
maintaining fiber.
[0033] FIG. 21a shows [data taken from] a transverse fiber grating
strain sensor embedded into an adhesive joint that was placed under
load. FIG. 21b shows data taken from the transverse fiber grating
strain sensor.
[0034] FIG. 22 is an illustration of a dual axis fiber grating
sensor embedded into an adhesive joint with its transverse strain
sensing axis aligned at -45 degrees.
[0035] FIG. 23 shows data taken from a transverse fiber grating
strain sensor embedded into an adhesive joint undergoing plastic
deformation.
[0036] FIG. 24 shows data of the displacement of the instumneted
adhesive joint from FIG. 23.
[0037] FIG. 25 is an illustration of a non-round coating on fiber
to prevent rolling and maintain desired orientation.
[0038] FIG. 26 is an illustration of forming a non-round coating
using heat.
[0039] FIG. 27 is a diagram of a prior art high-speed demodulation
system employing a grating filter to demodulate a grating
sensor.
[0040] FIG. 28a and FIG. 28b are diagrams showing different full
width half max spectra for grating filters allows for selection of
sensitivity and dynamic range.
[0041] FIG. 29a and FIG. 29b are diagrams showing how a change in
the periodic spacing of the perturbations of the index of
refraction, or grating spacing, changes the spectral position of
the grating.
[0042] FIG. 30a and FIG. 30b are diagrams showing the bending of a
simply supported beam to induce tension or compression in an
attached or embedded grating.
[0043] FIG. 31a and FIG. 31b are diagrams showing a cantilever
configuration for inducing tension or compression in an attached or
embedded grating.
[0044] FIG. 32 is a diagram showing the stretching or compressing
of a beam with force (F) to induce tension or compression in
grating.
[0045] FIG. 33 is an illustration of a tunable grating filter
requiring only one direction of tuning as the initial filter
wavelength is lower than that of the sensor allowing it to be tuned
into the range of the sensor.
[0046] FIG. 34 is an illustration of a tunable grating filter
employing a grating in a tube to control the amount of strain
transferred to the grating for a given displacement and allowing
for tuning in both directions if the fiber is pre-tensioned in the
tube and the grating is stretched or relaxed.
[0047] FIG. 35a and FIG. 35b are diagrams showing the application
of tension or compression to surface mounted or embedded fiber
grating through a pressure (P) differential across the
diaphragm.
[0048] FIG. 36a and FIG. 36b are diagrams showing the deflection of
a beam using a threaded stud to induce strain (positive or
negative) in a grating.
[0049] FIG. 37 is a photograph of the exterior of prototype with
fiber optic connections and knob on top to turn a threaded stud and
deflect a beam used to put tension and compression on the fiber
grating.
[0050] FIG. 38 is a photograph of the interior of prototype showing
threaded stud, beam, and beam supports.
[0051] FIG. 39 is a diagram showing a beam with multiple color
grating filters to filter different color grating sensors.
[0052] FIG. 40 is a diagram showing an adjustable comb filter.
[0053] FIG. 41 is a diagram showing a series of beams with attached
grating filters at different wavelengths to form an adjustable comb
filter.
[0054] FIG. 42 is a diagram showing a configuration where adjusting
each filter independently with a knob-beam configuration is
possible.
[0055] FIG. 43 is a diagram of a tunable grating filter based on
thermal tuning.
[0056] FIG. 44 is a diagram showing multiplexing of the high speed
demodulation system by introducing a time delay.
[0057] FIG. 45 is a diagram showing splitting the dual peak
structure of a dual axis grating to two individual peaks.
[0058] FIG. 46 is a diagram showing the use of polarization
controllers to separate out the two polarization states associated
with a dual axis (transverse) grating sensor.
[0059] FIG. 47 is a diagram of an alternative design where the
polarization controllers and polarizing fiber are placed before the
last beam splitters to reduce errors associated with inconsistent
polarization states in the filtered and reference legs.
[0060] FIG. 48 is a diagram showing the use of polarization
maintaining (PM) fiber and beam splitters in conjunction with
polarizers to control polarization states.
[0061] FIG. 49 is a diagram showing multiplexing of the transverse
gratings by combining two light sources and splitting each
wavelength to separate demodulators.
[0062] FIG. 50 is a diagram showing a "Cascading" configuration
where beam splitters are used to divide the reflected light from
the sensors among the separate demodulators.
[0063] FIG. 51 is a diagram showing the alternate location of
detectors.
[0064] FIG. 52 is a diagram showing another alternate location of
detectors to eliminate background light levels compared to FIG.
50.
[0065] FIG. 53 is a diagram showing another method to demodulate
several in line fiber grating sensors. This system also provides
the capability of an absolute measurement by providing a reference
detector.
[0066] FIG. 54 is a diagram showing an alternate configuration with
reference detectors on each leg.
[0067] FIG. 55 is a diagram showing how gratings written into beam
splitters can be employed to efficiently multiplex a high speed
fiber grating demodulation system.
DETAILED DESCRIPTION OF THE INVENTION
[0068] In the present invention, environmental sensing systems
based on fiber gratings are described. The environmental grating
sensors may be written onto ordinary single mode or birefringent
fiber. For the case where the environmental sensor is subjected to
a transverse load, it will behave differently depending on if it is
written onto ordinary single mode fiber or onto birefringent fiber.
To further increase the sensor's response to a transverse load,
voids such as sideholes may be introduced into the fiber. FIG. 1
shows a prior art transverse fiber grating sensor written onto
optical sidehole fiber as described in U.S. Pat. Nos. 5,828,059 and
5,841,131. The sidehole transverse fiber grating sensor 1 consists
of a length of sidehole fiber 3 that may have sideholes 5. When a
grating 7 is written onto the core 9 of the sidehole fiber 3, a
single distinct spectral peak results. The sideholes 5 in the fiber
may increase the sensor's 1 response to transverse strain.
[0069] Gratings written onto some sidehole fiber or ordinary single
mode fiber will reflect a single spectral peak in the unloaded
case. As the grating on some sidhole or single mode fiber is
transversely loaded, the spectral peak will begin to broaden and
eventually split as birefringence is induced in the fiber from the
external load. FIG. 2 shows a typical spectral response to
transverse loading for the case of a single grating written onto
non birefringent optical fiber, such as some sidehole fiber. In the
unloaded case 51, a single spectral peak results. As the transverse
load on the fiber sensor increases, the spectral peak 53 begins to
broaden. With further increasing load, the spectral peak begins to
split into two distinct spectral peaks 55.
[0070] For the case where a grating is written onto birefringent
fiber, two spectral peaks are reflected in the unloaded case, one
for each polarization state. As the grating written onto
birefringent fiber is transversely loaded, the spacing between the
two spectral peaks will change.
[0071] FIG. 3 shows a typical spectral response to transverse
loading for the case of a single grating written onto birefringent
optical fiber. In the unloaded case, two spectral peaks 101 result
with a peak separation 103. As the transverse load increases, the
separation of the two peaks 105 will increase. With further
increasing transverse loading, the spectral peak separation 107
will further increase.
[0072] FIG. 4 shows a chemical sensor based on transverse loading
of a strain sensor based on a single grating or multiple gratings
written onto birefringent or non-birefringent fiber. The chemical
sensor 151 consists of a chemical sensitive coating 153 that
expands in the presence of the target chemical to be sensed, such
as moisture. As the chemical sensitive coating 153 expands, it
exerts a force onto some stiff plates 155. The outward expansion is
prevented by clamps 157 and 159. This directs the force into the
grating sensor 161. The effective result is a transverse strain
impending on the grating sensor 161 in the presence of the target
chemical. The stiff plates 155 provide a more even loading on the
fiber as the chemical sensitive coating 153 expands. The relatively
large exposed area of the chemical sensitive coating 153 increases
the sensitivity and response time of this chemical sensor.
[0073] FIG. 5 shows another variation of a chemical sensor where a
series of chemical sensitive coatings are cascaded together to
increase the amount of force directed into the fiber grating sensor
to increase sensitivity. This variation of the chemical sensor 201
consists of multiple stacks of chemically sensitive coating 203
with stiff plates 205. As these multiple sets of chemical sensitive
coatings 203 expand in the presence of the target chemical, their
combined force is directed into the fiber grating sensor 207. By
controlling the quantity n and m of the stacks, the sensitivity of
the chemical sensor 201 can be controlled.
[0074] FIG. 6 shows another variation of a chemical sensor where
the grating sensor is embedded into a piece of composite tow where
the force on the fiber is transverse. The chemical sensor 251
consists of a fiber grating sensor 253 that is formed from one or
two gratings written onto birefringent or non-birefringent optical
fiber. The fiber grating sensor 253 is embedded into a piece of
composite tow 255 which can have many functions such as isolating
the fiber grating sensor 253 from chemicals that would be damaging
to the optical fiber and keeping the orientation of the fiber
grating sensor 253 correct for the case where birefringent fiber is
used. The composite tow piece 255 is surrounded by stiff plates 257
and chemical sensitive coating 259 (or affinity coating.) As the
chemical sensitive coating expands or shrinks in the presence of
the target chemical or chemicals, the force exerted on the tow 255
changes and hence the strain on the fiber grating sensor 253
allowing a measurement to be made.
[0075] FIG. 7 shows another variation of a chemical sensor 301
where the fiber grating sensor 303 is embedded into a composite
part 305 with some optimal geometry for the chemical sensitive
coating 307 (or affinity coating) to maximize the strain on the
fiber grating sensor 303 in the presence of the target chemical or
chemicals.
[0076] FIG. 8 shows another variation of a chemical sensor based on
a v-groove configuration. This chemical sensor 350 consists of a
fiber grating sensor 353 that is formed from a single or multiple
gratings on birefringent or non-birefringent fiber placed into a
v-groove 355. This plate keeps the fiber in place and can help
maintain the proper orientation 357 of the fiber if a grating in
birefringent fiber is used. As the chemical sensitive coating 359
expands in the presence of the target chemical or chemicals, it
exerts a force on the top plate 361 which transfers the force to
the fiber grating sensor 353.
[0077] FIG. 9 shows another variation of a chemical sensor based on
a double v-groove configuration. The chemical sensor 401 consists
of a double v-groove plate 403 that holds both the fiber grating
sensor 405 and a dummy fiber 407 of the same diameter as the fiber
grating sensor but without a grating element. This configuration
helps to reduce the rocking effect of the top plate 409 on top of
the fiber grating sensor 405 to provide a more consistent loading
as the chemical sensitive coating 411 expands in the presence of
the target chemical or chemicals. The v-grooves in plate 403 help
keep the fibers in place and keep the fiber grating sensor 405
oriented if a birefringent fiber is used.
[0078] FIG. 10 shows another variation of a chemical sensor based
on a v-groove configuration. The chemical sensor 451 consists of a
v-groove plate 453 and side channels 455. The side channels can
help keep the top plate level for more consistent loading on the
fiber grating sensor 459 as the chemical sensitive coating 461
expands in the presence of the target chemical or chemicals. The
v-groove plate 453 helps keep the fiber in place and keeps the
fiber grating sensor 459 oriented if a birefringent fiber is
used.
[0079] FIG. 11 shows another variation of a chemical sensor based
on a multiple v-groove configuration to support multiple sensing
points. The chemical sensor 501 consists of multiple v-groove
plates 503 and side channels 505 that allow for multiple fiber
grating sensors 507 to be loaded as the chemical sensitive coating
509 expands against the plates 511. This configuration can extend
the sensing range and provide better accuracy by comparing the
multiple grating sensors 507 to each other.
[0080] FIG. 12 shows how a beveled plate 551 may be used to
increase the surface area of the chemical sensitive coating 553 and
increase the wicking action of the target chemical or chemicals
into the coating. This could increase sensitivity and decrease
response times of the chemical sensor.
[0081] FIG. 13a, FIG. 13b, and FIG. 13c show plates of differing
permeability 601 and holes 603 or slots 605 can be used to increase
the volume and rate of absorption of the target chemical into the
chemical sensitive coating.
[0082] FIG. 14 shows another variation of a chemical sensor 651
based on a flexible plate 653 to transfer the load from the
chemical sensitive coating 655 to the fiber grating sensor 657
which can consist of one or more gratings written onto birefringent
or non-birefringent fiber. The multiple v-groove plate 659 can hold
multiple dummy fibers 661 to provide different loading schemes for
the flexible plate 653. The flexible plate 653 allows for
inconsistent swelling of the chemical sensitive coating 655.
[0083] FIG. 15 shows another variation of a chemical sensor where
the chemical sensitive coating is placed directly on the fiber. The
chemical sensor 701 consists of a fiber grating sensor 703 that can
consist of a single or multiple gratings on birefringent or
non-birefringent fiber. A chemical sensitive coating 705 exerts a
transverse force on the fiber grating sensor 703 as it swells in
the presence of a target chemical or chemicals.
[0084] FIG. 16 shows another variation of a chemical sensor where
composite tow that is reactive to a target chemical is used to
transversely load the fiber grating sensor. The chemical sensor 751
consists of chemically reactive composite tow 753 which expands or
shrinks in the presence of the target chemical or chemicals to
transfer a load to the fiber grating sensor 755. The fiber grating
sensor 755 can consist of one or more gratings on birefringent or
non-birefringent fiber. A void 757 can be used to provide clean
transverse loads on the fiber grating sensor 755.
[0085] The above descriptions describe a transverse strain applied
to the grating sensor on the presence of a target chemical such as
moisture. Another application to the transverse strain sensing
capability of the fiber grating written onto either ordinary single
mode fiber or birefringent fiber is the direct measurement of
transverse strain and strain gradients when embedded into a
structure such as an adhesive joint.
[0086] One key problem facing structural designers is the ability
to be able to monitor the structural integrity of adhesive joints.
While these joints are used in many types of construction there is
very strong motivation to use these in aerospace applications to
improve strength and reliability while lowering construction costs
and overall weight. FIG. 17 is a diagram of a wing structure 2001
that may be made of lightweight composite material. The wing 2001
is made up of sections that may be adhesively bonded and strings of
fiber grating sensors 2003, 2005 and 2007 can monitor these
bonds.
[0087] FIG. 18 shows an adhesive bond 2051 that joins two parts
2053 and 2055. When the parts 2053 and 2055 are pulled apart by the
forces 2057 and 2059 a shear load is formed along the line 2061. A
multi-axis fiber grating sensor 2063 can be placed along the length
of the adhesive bond 2051 with its traverse axes 2065 and 2067
aligned along the shear line 2061 and orthogonal to it so that
shear strain induced in the bond may be measured. While the diagram
of FIG. 18 shows the fiber grating sensor 2063 place interior to
the adhesive joint 2051 other positions are possible.
[0088] FIG. 19 shows the placement of three fiber grating sensing
fibers 2101, 2103 and 2105 in the adhesive bond 2107, between the
bonded materials 2109 and 2111. Note that the fiber grating sensor
2101 is placed well into the adhesive bond 2107 while the fiber
grating sensor 2103 is placed near the edge and the fiber grating
sensor 2105 is placed in the exterior. When an adhesive bond starts
to fail under shear load it usually starts on the edge. So the
placement of the fiber grating 2105 just exterior to the adhesive
bond 2107 is in the area where failure is likely to first occur.
This arrangement is also useful for enabling a system that could be
used as a failure warning mechanism for existing adhesive bonds as
an exterior bead of adhesive could be added and oriented fibers
placed at the edge of a bond to provide a health monitoring system
as a retrofit to existing structures or to simplify fabrication of
new structures.
[0089] FIG. 20a and FIG. 20b are diagrams that are used to
illustrate the action of a multi-axis fiber grating sensors that is
placed inside of a material that is subject to strains and eventual
failure. In particular this would be the case of an adhesive bond
that is strained until it fails. In FIG. 20a a multi-axis fiber
grating sensor 2151 with transverse sensing axes 2153 and 2155 is
subject to a uniform loading force 2157 along the axis 2153.
When-this type of uniform transverse loading occurs two spectral
output peaks 2159 and 2161 occur that are smooth curves whose
central wavelengths shift so that the two peaks 2159 and 2161 move
apart or together with wavelength difference. FIG. 20b illustrates
the case of the fiber optic grating sensor 2151 when the transverse
loading force 2171 is not uniform. This would be the case for
example when an adhesive bond under load starts to break apart
along the line of the axis 2153. In this case the spectral peak
2161 in FIG. 20b will split into two wavelength peaks 2173 and
2175. The spectral separation between the peaks 2173 and 2175
provides a quantitative means to measure the difference in load
along the axis 2153 generated by the force 2171 that consists of
the load regions 2177 and 2179. The intensity of the peaks 2173 and
2175 provide a means to determine the lengths of the load regions
2177 and 2179. In the case of FIG. 20b the regions are nearly equal
in length resulting in the two peaks being nearly equal in
intensity.
[0090] FIG. 21b is a diagram showing experimental results that were
obtained by using a multi-axis fiber grating to monitor an adhesive
bond. Additional experimental data on joints that were tested using
this technology can be found in W. L. Schulz, F. Udd, M. Morrell,
J. Seim, I. Perez, A. Trego, "Health Monitoring of an Adhesive
Joint using a Multiaxis Fiber Grating Strain Sensor System", SPIE
Proceedings, Vol. 3586, p. 41, 1999. In FIG. 21a the multi-axis
fiber grating sensor 2201 is oriented at 45 degrees relative to the
adhesive bond 2203, and the plates 2205, 2207, 2209, and 2211. The
fiber grating sensor 2201 is placed at the edge of the adhesive
bond 2203 so that it can be used to predict the onset of failure
during loading. The graph shown in FIG. 21b illustrates the
spectral reflective output of the multi-axis fiber grating sensor
2201 of FIG. 21a as a function of load being applied by an Instron
machine to the plates 2205 and 2207 that are being pulled apart.
Note that after a certain load level is applied of approximately
1800 pounds the two major spectral peaks start to move apart with
continuing increases in load. At about 2400 pounds the major
spectral peak 2251 splits into the two peaks 2253 and 2255. The
spectral separation 2257 between these two peaks 2253 and 2255 is
approximately 0.2 nm. Since the response of the multi-axis fiber
grating sensor in the transverse direction is approximately a
factor of 3 lower than in the axial direction a change of 0.2 nm
corresponds to a change of about 600 micro-strain. The intensity of
the two split peaks 2253 and 2255 being nearly equal means that
along the axis of shear strain (along which one of the transverse
axes of the multi-axis fiber 2201 is aligned as shown in FIG. 21a)
about half the fiber grating length has been unloaded by about 600
micro-strain while the other half of the grating remains at the
higher load level. Since the fiber grating used in this case is
about 5 mm in length this means that approximately 2 mm of the
fiber grating sensor 2201 along the shear strain axis has been
unloaded due to a change in the adhesive bond 2203. As the adhesive
bond 2203 is subject to increasing load additional peaks arise with
greater intensity indicating additional breakage and the overall
spectral profile 2257 moves toward longer wavelengths indicating
axial loading is occurring. Thus FIG. 21a and FIG. 21b illustrate
the ability of a multi-axis fiber grating sensor 2201 to measure
transverse strain which because of its orientation at 45 degrees
measures shear strain in the adhesive bond 2203. This figure also
illustrates the ability to measure changing transverse strain
gradients indicative of break up of the adhesive bond 2203 and
axial strain changes that occur in this example before failure of
the bond 2203.
[0091] In addition to monitoring break up of adhesive bonds and
failure it is possible to use multi-axis fiber grating sensors to
monitor plastic deformation of an adhesive bond under cycling. FIG.
22 shows the position of the multi-axis fiber grating sensor 2301
that is oriented at -45 degrees relative to the adhesive bond 2303
and the plates 2305, 2307, 2309 and 2311. As the plates 2311 an
2309 and pulled apart with increasing force and then unloaded the
multi-axis fiber grating sensor 2301 can be used to monitor the
adhesive bond 2303 in the neighborhood of its placement. FIG. 23 is
a graph showing the displacement of the major spectral peaks during
a cycle of the adhesive bond 2303. The spectral profile 2351 shows
the original spectrum after the multi-axis fiber grating sensor
2301 after placement in the adhesive bond 2303 but before loading.
In this particular case after the adhesive bond 2303 was cycled it
did not fail but the unloaded spectra after the cycle 2353 reflects
a change in the strain fields interior to the adhesive joint 2303.
FIG. 24 illustrates the displacement of the plates 2309 and 2311 by
an Instron loading machine during testing. Note that the adhesive
joint 2303 has been plastically deformed during this cycle as was
expected as the cycle was beyond the load expected to fail the
part. The spectral profiles of FIG. 23 illustrate this process.
[0092] In the above sensor configurations, one possible
configuration is to use one or more fiber gratings written onto
birefringent fiber. The polarization axes associated with the
birefringent fiber require that the fiber grating sensor be placed
in a known orientation in order to maximize the sensitivity of the
sensor's response to a transverse load. FIGS. 25 and 26 describe
one possible method of controlling the orientation of a fiber
grating sensor written onto birefringent fiber.
[0093] FIG. 25 shows a method to control the orientation of a fiber
grating sensor based on birefringent fiber. In this case, a
non-symmetric coating 801 is placed over the fiber grating sensor
803. The orientation of the polarization axes of the fiber grating
sensor 805 can be controlled by placing the flats 807 of the
coating in the desired orientation.
[0094] FIG. 26 shows how the non-symmetric coating of FIG. 25 can
be manufactured. The process begins with a fiber of known
orientation 851 with a round fiber coating 853 that will melt with
enough heat placed between two plates 855. As the plates are heated
857, the coating 859 will begin to melt and flow and form flats 861
where the coating touches the plates 855.
[0095] In many areas where environmental sensing is required, there
is a desire for high sensitivity and multiple sensing points. For
this reason, a demodulation system with high sensitivity and a
large multiplexing potential is needed. In the figures below,
several systems are described that enhance the capability of a
fiber grating demodulation system using spectral filters described
in U.S. Pat. Nos. 5,380,995 and 5,397,891.
[0096] FIG. 27 shows a prior art fiber grating demodulation system
using spectral filters described in U.S. Pat. Nos. 5,380,995 and
5,397,891. The fiber grating demodulation system consists of a
broadband light source 3001 that directs broadband light through a
beam splitter 3003 and to a fiber grating sensor 3005. The fiber
grating sensor 3005 reflects a spectral peak based on the strain on
the grating that travels back through beam splitter 3003 and is
then directed to a second beam splitter 3007 where it is split
between lines 3009 and 3011. The spectral peak traveling along line
3009 travels through a fiber grating filter 3013 that converts the
spectral information into an amplitude based signal. The spectral
peak then travels from the grating filter 3013 to the detector
3015. The spectral peak in line 3011 travels directly to the
high-speed detector 3017 to provide a reference measurement. The
detector then outputs two voltages 3019 and 3021 that can be
acquired by a data acquisition system 3023.
[0097] FIG. 28a and FIG. 28b show typical spectral profiles from a
grating written onto non-birefringent fiber. This is one
possibility for the fiber grating filter described in FIG. 25. In
order to adjust the sensitivity of the fiber grating filter,
gratings of different widths may be used to control the slope of
the spectral profile. If a narrower grating is used as a filter,
its spectral profile 3051, shown in FIG. 28a, will give more
sensitivity due to its steeper slope, but will give less dynamic
range for the sensor to sweep across. If a wider grating is used as
a filter, its spectral profile 3053 will give a shallower slope,
for decreased sensitivity, but a wider dynamic range, shown in FIG.
28b.
[0098] FIG. 29a and FIG. 29b each show a typical response of a
fiber grating sensor to an axial load. The grating under no load
3101, shown in FIG. 29a, will have a grating spacing 3103 resulting
in a spectral peak at a lower center wavelength 3105. As the fiber
grating sensor is axially strained 3107, the grating spacing 3109
results in a spectral peak at a higher center wavelength 3111,
shown in FIG. 29b. This shows how the grating sensor will sweep
across the grating filter in the system described in FIG. 27.
[0099] When fiber grating sensors are installed onto or embedded
into structures, many times the initial strain state is different
than it was for the uninstalled sensor due to such mechanisms as
residual stress. This initial tensile or compressive force results
in the fiber grating sensor's initial spectral peak center to be at
a different wavelength than the unstrained sensor. Referring back
to the demodulation system of FIG. 27, if the spectral filter does
not match up spectrally with the fiber grating sensor, then there
will be no measurable change in amplitude as the sensor is
modulated. For this reason, a tunable grating filter may be needed
to ensure that the spectral filter matches up with the initial
state of the installed sensor. The following figures describe
methods for straining a fiber grating and thus providing a tunable
grating filter.
[0100] FIG. 30a and FIG. 30b show] a tunable filter concept where a
fiber grating sensor is attached to or embedded into a simply
supported 3131 flexing beam 3133 above the neutral axis of the
beam. As the beam is bent up 3135, FIG. 30a, or down 3137, FIG.
30b, the grating on the beam will be subjected to tension or
compression allowing for a filter that can be tuned to both higher
and lower wavelengths. The beam can also be supported other ways,
such as fixed, etc.
[0101] FIG. 31a and FIG. 31b show a tunable filter concept
utilizing a bending beam with a grating attached onto or embedded
into the beam above the neutral axis of the beam. As the beam is
bent up 3151, FIG. 31a, or down 3153, FIG. 31b, the grating on the
beam will be subjected to tension or compression.
[0102] FIG. 32 shows a tunable filter concept utilizing a beam 3171
with a grating 3173 attached onto or embedded into the beam. As the
beam is stretched or compressed with a force 3175, the fiber
grating will be subjected to tension or compression and thus can be
tuned to higher or lower wavelengths.
[0103] FIG. 33 shows a tunable filter concept where a fiber grating
3181 is fixed at a point along its length 3183. A force 3185 pulls
on the grating to induce tension and thus a spectral shift to a
higher wavelength. The fiber grating 3181 is written at a lower
wavelength than is expected for the installed fiber grating sensor.
An example of this would be to use a fiber grating filter in this
configuration at 1297 nanometers for demodulating a fiber grating
sensor with nominal wavelength at 1300 nanometers. This would allow
for the tunable filter to match up with the fiber grating sensor by
only having to tune it in one direction.
[0104] FIG. 34 shows an extension to FIG. 33 where the fiber
grating 3201 is placed into a tube 3203 and fixed at either end of
the tube 3205, 3207. The tube is also fixed 3209. The length of the
tube 3211 can be varied to control the length of the sensor that is
being stretched by force F 3213 and thus control the amount of
strain on the fiber for a given displacement controlled by a
precision screw such as a micrometer or a picomotor such as the one
available from New Focus. This configuration could be a tension
only type of tunable filter similar to FIG. 33, or the fiber could
be pre-strained in the tube to allow for a wavelength shift in both
directions if the fiber was allowed to relax.
[0105] FIG. 35a and FIG. 35b show a tunable filter concept
utilizing a diaphragm 3221 with a fiber grating attached onto or
embedded into the diaphragm off of its neutral axis. With a
pressure differential on the diaphragm 3223, 3225 the diaphragm
will deflect up or down and put tension or compression on the fiber
grating.
[0106] FIG. 36a and FIG. 36b show an extension to the tunable
filter concept shown in FIG. 30. In this case, a threaded stud 3241
is threaded through a tap 3243 in the beam 3245. As the stud 3241
is turned the beam 3245 is flexed up or down based on the direction
of the turn.
[0107] FIG. 37 shows a picture of a prototype based on the concepts
described in FIGS. 30 and 36. Here the tunable grating filter is
enclosed in a box with an external knob to turn the threaded stud
inside. The optical ports 3247 allow access to both sides of the
grating to allow the filter to operate in transmission.
[0108] FIG. 38 shows a picture of the inside of the filter box of
FIG. 37. Here the beam 3261 with the attached grating can be seen
with the stud threaded 3263 through it.
[0109] FIG. 39 shows an extension of the tunable filter concept
where multiple gratings 3281,3283 of different wavelengths
3285,3287 are attached to or embedded into a beam with tuning
provided by bending or a push/pull force. This allows for the
potential of a single tunable filter handling multiple fiber
grating sensors at different wavelengths.
[0110] FIG. 40 shows the spectral profile 3301 of a series of
tunable gratings. If each spectral peak were tunable independently,
then a comb filter could be formed.
[0111] FIG. 41 shows a concept for the fiber grating comb filter
shown in FIG. 40. A series of multiple beams 3303 or other tuning
mechanisms each with a fiber grating 3305 of different wavelength
attached or embedded could be connected together to form the comb
filter.
[0112] FIG. 42 shows how the fiber grating comb filter could be
packaged and tuned. A series of knobs 3321 connected to the beams
with gratings at different wavelengths 3323 could be used to tune
each individual grating to a higher or lower wavelength to form the
desired comb profile. Optical ports 3325 would provide access to
both ends of the series of gratings.
[0113] FIG. 43 shows another concept for a tunable grating filter.
As a fiber grating responds similarly to heat as it does to strain
due to thermal expansion/contraction, a tunable filter based on
heating/cooling the fiber grating is feasible. A heat input 3341
would shift the grating filter 3343 to a higher wavelength. A heat
output 3345 or cooling would shift the grating filter to a lower
wavelength.
[0114] In addition to a tunable grating filter to support higher
sensitivity and multiplexing of the grating based sensor such as a
chemical sensor, additional schemes are described below that
further enhance the multiplexing potential of a fiber grating
sensor system.
[0115] FIG. 44 shows a modification of the demodulation system
described in FIG. 27 where multiplexing is enabled through the use
of time division multiplexing. The demodulation system 3361
consists of a pulsed broadband light source 3363 that directs a
spectral pulse 3365 into a beam splitter 3367 and is split into two
pulses 3369 and 3371. The pulse 3369 will arrive at the grating
sensor 3373 first and a spectral peak 3375 will be reflected back.
The spectral pulse 3371 will reach the grating sensor 3377 later
due to a time delay 3379 that could consist of a coil of fiber. The
grating sensor 3377 will then reflect a spectral peak 3381. The
spectral peak 3375 will reach the beam splitter 3367 first and be
split into two spectral peaks 3383 and 3385. Spectral peak 3383
will be directed back toward the light source 3363 and will have no
effect. Spectral peak 3385 will be directed toward a second beam
splitter 3387 that will split it into two spectral peaks 3389 and
3391. The spectral peak 3381 will reach the beam splitter 3367
after peak 3375 and will be split into two peaks that will follow
the same paths as spectral peaks 3383 and 3385, only they will be
delayed by the amount determined in the time delay 3379. This
configuration allows for multiple gratings sensors at the same
wavelength to be demodulated by one demodulation system with a
single spectral filter 3393.
[0116] In some of the demodulation cases described above, only a
single spectral peak being reflected from the grating sensor can be
demodulated. The following figures describe methods for utilizing
this same demodulation system for the case of gratings written onto
birefringent fiber where there are multiple peaks per sensor, refer
to U.S. Pat. Nos. 5,591,965 and 5,828,059.
[0117] FIG. 45 shows a typical spectral profile 3401 for a grating
written onto birefringent fiber. The profile consists of two peaks
3403 and 3405 associated with the polarization states of the
birefringent fiber onto which the grating is written. In order to
utilize the above described high speed demodulation system, these
polarization peaks 3403 and 3405 can be separated 3407 into two
separate peaks 3409 and 3411 that are compatible with the high
speed demodulation system.
[0118] FIG. 46 shows a demodulation system utilizing the concept of
FIG. 45 to demodulate a grating written onto birefringent fiber
with the demodulation system employing a spectral filter described
previously. The broad band light source 3421 directs a broad band
spectral profile 3423 into a beam splitter 3425 which splits the
broad band profile 3423 into two broadband profiles 3427 and 3429.
The profile 3429 can be dumped (ensuring no back reflections) or
directed toward another grating sensor. The profile 3427 is
directed toward a fiber grating sensor 3431 written onto
birefringent fiber where two spectral peaks 3433 and 3435
associated with the polarization axes of the birefringent fiber
will be reflected. These peaks are then directed toward the beam
splitter 3425 and directed toward a second beam splitter 3437 and
split into legs 3439 and 3441. The two peaks traveling along leg
3439 are directed into beam splitter 3443 and split into legs 3445
and 3447. The two peaks in leg 3445 are directed into a
polarization controller 3449. A length of polarizing fiber 3453 is
used to ensure that one of the polarization states is blocked. The
peak single 3455 is then directed into a spectral filter 3457 and
converted into an amplitude based measurement measurable by a
detector 3459 as described in FIG. 27. The leg 3447 provides the
reference leg described in FIG. 27. The leg 3441 directs the two
peaks associated with the two polarization states into a beam
splitter 3461 that splits into two legs 3463 and 3465. Leg 3463
directs the two peaks into a polarization controller 3467. A length
of polarizing fiber 3471 is used to ensure that one of the peaks is
blocked. The peak 3473 is then directed into a spectral filter 3475
and converted into an amplitude based measurement measurable by a
detector 3477 as described in FIG. 27. The leg 3465 provides the
reference leg described in FIG. 27. To ensure that the polarization
controllers and polarizing fibers are blocking the correct
polarization peaks, a simple calibration could be performed by
loading the fiber grating in transverse and noting whether or not
the signals on the respective detectors change as expected.
[0119] FIG. 47 shows another method to separate the polarization
states of the grating written onto birefringent fiber. This method
places the polarization controllers before the beam splitter that
splits the spectral data between the filtered and reference leg
reducing errors associated with inconsistent polarization states in
the filtered and referenced legs. A broadband light source 3481
outputs a broadband profile 3483 to a beam splitter 3485 that
splits the profile 3483 into two legs 3487 and 3489. The leg 3489
is dumped or can be connected to another grating sensor. The leg
3487 guides the broadband light to a fiber grating sensor 3491 that
consists of a grating written onto birefringent fiber that reflects
two spectral peaks 3493 and 3495 each associated with a
polarization state of the birefringent fiber. These peaks 3493 and
3495 are then directed to the beam splitter 3485 and directed 3497
into a beam splitter 3499 that splits into legs 3501 and 3503. The
two peaks in leg 3501 are directed into a polarization controller.
Polarizing fiber 3509 ensures that one of the polarization states
is dropped. The peak 3511 is then directed in to a beam splitter
3513 that splits into two legs 3515 and 3517. Leg 3515 directs the
single peak associated with one of the polarization states of the
fiber grating sensor written onto birefringent fiber into a
spectral filter 3519 that converts the spectral information into an
amplitude based signal measurable by a detector 3521. The leg 3517
provides the reference leg. The two peaks in leg 3503 are directed
into a polarization controller 3523. A length of polarizing fiber
3527 ensures that one of the polarizing states is dropped. The peak
3529 is then directed into a beam splitter 3531 that splits into
two legs 3533 and 3535. Leg 3533 directs the single peak associated
with one of the polarization states of the fiber grating sensor
written onto birefringent fiber into a spectral filter 3537 that
converts the spectral information into an amplitude based signal
measurable by a detector 3539. The leg 3535 provides the reference
leg.
[0120] FIG. 48 shows an alternative system where polarization
maintaining fiber is used throughout most of the system along with
polarization maintaining beam splitters so that the two
polarization states are each directed to the appropriate
demodulator filter set. A broad band light source 3561 directs
broad band light 3563 into a polarization maintaining beam splitter
3565 that splits the broadband light 3563 into two parts 3567 and
3569. Broadband light 3569 is dumped or can be connected to another
fiber grating sensor. Broadband light 3567 is directed along the
fiber that is placed into a tube 3571 that provides strain relief
for the fiber going into a part 3572 to a fiber grating sensor 3573
written onto birefringent fiber that reflects two peaks 3575 and
3577 associated with each polarization state of the birefringent
fiber. The peaks 3575 and 3577 are directed to beam splitter 3565
and then directed to polarization maintaining beam splitter 3581
that splits into two legs 3583 and 3585. The leg 3583 directs both
peaks associated with the polarization axes of the fiber grating
written onto birefringent fiber to a length of polarizing fiber
3587 that is oriented to block one of the polarization states. The
fiber and beam splitters after this length of polarizing fiber 3587
does not need to be polarization maintaining. The resulting single
peak from 3587 then travels to a beam splitter 3589 and is split
into legs 3591 and 3593. Leg 3591 directs the single peak to a
fiber grating filter 3595 that converts the spectral information
into an amplitude based signal measurable by a detector 3597. The
3593 leg forms the reference leg. The leg 3585 directs both peaks
associated with the polarization axes of the fiber rating written
onto birefringent fiber to a length of polarizing fiber 3599 that
is oriented to block one of the polarization states different from
that of 3587. The fiber and beam splitters after this length of
polarizing fiber 3599 does not need to be polarization maintaining.
The resulting single peak from 3599 then travels to a beam splitter
3601 and is split into legs 3603 and 3605. Leg 3603 directs the
single peak to a fiber grating filter 3607 that converts the
spectral information into an amplitude based signal measurable by a
detector 3609. The 3605 leg forms the reference leg.
[0121] FIG. 49 shows a method to add multiplexing capability to the
system shown in FIG. 48 by employing two broadband light sources
and two gratings written at different wavelengths. In this case,
two broadband light sources 3621 and 3623 of different central
wavelengths are combined using a wavelength division multiplexer
3625. The resulting two broadband profiles are directed into leg
3627 and to a beam splitter 3629 that splits into two legs 3631 and
3630. Leg 3630 is dumped or could be connected to a fiber grating
sensor. Leg 3631 directs the two broadband profiles to a grating
sensor 3635 written onto birefringent fiber and reflecting two
peaks 3637 and 3639 each associated with the polarization axes of
the birefringent fiber. The throughput of the grating sensor 3635
is directed to another grating sensor 3641 written onto
birefringent fiber at a different wavelength than grating sensor
3635 and reflecting two peaks 3643 and 3645 each associated with
the polarization axes of the birefringent fiber. The resulting four
peaks 3637, 3639, 3643, and 3645 are then directed to a beam
splitter 3629 and directed to a wavelength division multiplexer
(providing lower loss) or a beamsplitter 3647 that divides the four
peaks into two pairs associated with the center wavelengths of the
broadband light sources 3621 and 3623. One pair of peaks travels
along leg 3649 into a demodulation system 3653 similar to that
described in FIG. 48. The other pair of peaks travels along leg
3651 into a demodulation system 3655 similar to that described in
FIG. 48. The approach of FIG. 49 could be extended to large numbers
of sensors by using the wavelength division multiplexing element
3647 to divide the spectrum into discrete packets for each fiber
grating sensor, demodulation subsystem combination.
[0122] In order to multiplex a large number of fiber grating
sensors using wavelength division multiplexing while retaining high
speed characteristics and sensitivity it would be highly desirable
to have the lowest possible loss system available.
[0123] FIG. 50 shows a system that may be used to multiplex fiber
optic gratings at high speed using low cost 2 by 2 fiber couplers.
There are different means to operate the system shown in FIG. 50.
As an example the light source 3801 could be a broadband light
source such as a light emitting, superradiant laser diode or doped
fiber light source (erbium doped light sources being currently most
common), which could be used to illuminate a series of fiber
grating sensors spaced in wavelength simultaneously. The light
source 3801 could also be a tunable light source such as a tunable
laser diode that could be used to spectrally scan the string of
fiber grating sensors. Returning to FIG. 50, the light source 3801
emits a beam of light that is coupled into one end of the fiber
coupler 3805 (bulk optic components or integrated optic
beamsplitters could be used, currently the losses associated with
these devices are higher and they are not as cost effective). The
light beam 3803 is then split by the beamsplitter 3805 into a light
beam 3807 that exits the system in FIG. 50 but it could also be
used to illuminate another set of fiber grating sensors on a second
fiber line. The second split portion of the light beam 3803 is the
light beam 3809 that is directed toward the fiber grating sensor
3811 centered about the wavelength .lambda..sub.1. A portion 3819
of the light beam 3809 is reflected by the fiber grating sensor
3811. The spectral change of the light beam 3819 is indicative of
the environmental state of the fiber grating. The light beam 3819
then traverses the fiber beamsplitter 3805 a second time and a
portion of it is directed to the beamsplitter 3823 where it is
split again by the beamsplitters 3825 and 3827 eventually resulting
in the light beam 3821 hitting the beamsplitter 3829. The light
beam 3809 then proceeds past 3811 to the fiber grating sensor 3813
that is centered about the wavelength .lambda..sub.2. A portion
3831 of the light beam 3809 is reflected off the fiber grating
sensor 3813 and is split by the beamsplitters 3805, 3823, 3825 and
3827 to form the light beam 3835 that is directed toward the
beamsplitter 3829. In a similar manner portions of the light beam
3809 are reflected from the fiber grating sensors 3815 centered
about .lambda..sub.3 and 3817 centered about .lambda..sub.8. The
net result is that at the beamsplitter 3829 there is a light beam
consisting of reflections off the series of fiber grating sensors
3811, 3813, 3815 and 3817 divided by the action of the
beamsplitters 3805, 3823, 3825 and 3827. A similar light beam 3837
falls onto the beamsplitter 3839. Analogous combination light beams
3841 and 3843 fall onto the beamsplitters 3845 and 3847
respectively.
[0124] When the light beam 3849 corresponding to reflections off
all the fiber grating sensors 3811, 3813, 3815, 3817 falls onto the
beamsplitter 3829 it splits into the light beams 3851 and 3853. The
light beam 3851 falls onto the output detector 3855 whose output
signal acts as reference. The light beam 3853 passes through the
fiber grating filter 3857 that acts to modulate the spectral signal
reflected from the fiber grating sensor 3811. The light beam 3859
passing through the fiber grating filter 3857 then falls onto the
output signal detector 3861. Note that the output signal from
detector 3861 contains a constant component associated with the
reflections off all the other fiber grating sensors in the system
in addition to that of 3811. The result is an offset for the output
signal that becomes increasingly large with additional fiber
grating sensors. Similar considerations apply to the beamsplitter,
fiber grating filter detector sets 3863, 3865, 3867, 3869, 3871,
3873 and 3875.
[0125] Another approach to the fiber grating sensor system shown in
FIG. 50 is to have the light source 3801 be a tunable laser. In
this case each fiber grating sensor 3811, 3813, 3815, 3817 is
illuminated in sequence. The only variation in intensity as the
light source is swept corresponds to the filter/detector pair
corresponding to the illuminated grating. As an example when fiber
grating sensor 3811 is swept the reflected light beam from 3811 is
directed through the series of beamsplitters 3805, 3823, 3825, 3827
and 3829 to the fiber grating filter 3857 which in turn modulates
the swept signal and by comparing the output of 3861 to 3855 the
wavelength may be determined. Similarly the output of the fiber
sensor grating 3813 can be read out by the optics/detector set
3857, fiber sensor grating 3815 by the optics/detector set 3865 and
3817 by the optics/detector set 3875. While one fiber sensor
grating is being readout by the tunable laser 3801 the other
optics/detector sets have a fixed ratio.
[0126] FIG. 50 illustrates the case where two by two couplers are
used. As shown in FIG. 51 it is also possible to use 1 by n
couplers to achieve similar results. In this case the same light
source 3801 is used to illuminate the sequence of fiber grating
sensors 3811, 3813, 3815 and 3817. The reflected light beams from
these fiber grating sensors are then directed to the 1 by n
beamsplitter 3901 into n light beams each of which is directed
through a fiber grating filter and onto the output detectors
corresponding to each fiber grating sensor. In the simplest case
the spectral signal would be modulated directly and not referenced.
Reference detectors such as 3903 could be added with reference
beamsplitters such as 3905 to compensate for system level
fluctuations. An alternative configuration would be to place a
reference detector 3909 at one of the output legs of the two by two
beamsplitter 3907.
[0127] FIG. 52 shows a configuration of a multiplexed fiber grating
sensor system similar to that shown in FIG. 50 where instead of the
output signal detectors monitoring the optical beams passing
through the filters the light is reflected. This configuration
eliminates cross talk between the fiber gratings. As an example the
reflection from the fiber grating sensor 3811 is modulated only by
the fiber grating filter 3955 which is designed to modulate light
only about the center frequency of the fiber grating sensor 3811.
The modulated light is then reflected to the output detector 3951.
In a similar manner the fiber grating filter 3957 acts only to
modulate the reflected light from the fiber grating sensor 3813 and
in turn directs its modulated output light signal to the detector
3953. The configuration in FIG. 49 could be modified to replace the
two by two couplers with a 1 by n coupler in direct analogy to FIG.
51.
[0128] FIG. 53 illustrates a system comprised of fiber gratings in
a single fiber line with a series of fiber beamsplitters. This
system can be operated in a number of different ways. In the first
case consider the light source 4001 to be a broadband light source
that might be a light emitting diode or a superradiant diode. The
light source 4001 couples the light beam 4003 into the beamsplitter
4005. A portion of the light beam 4007 is directed through a series
of fiber gratings 4011, 4013, 4015 . . . 4017 in the optical fiber
line 4119. Another portion of the beam 4003 that is split by the
beamsplitter 4005 is split off into the light beam 4009 that exits
the system in FIG. 53 but alternatively could be used to support
another line of fiber gratings. The reflected spectra from the
fiber gratings 4011, 4013, 4015 . . . 4017 return to the
beamsplitter 4005 and a portion of these spectra are directed along
the output fiber 4021 as the light beam 4023. The light beam 4023
passes to the first fiber beamsplitter 4025 and a portion of it
4027 is split off to the reference detector 4029 along the fiber
4031. The signal from the detector 4029 is used to monitor the
overall light level of the light source and components up to this
point in the system. The second portion of the beam 4023, 4033 is
directed to the fiber grating filter 4035 that has a wavelength
designed to match that of fiber grating sensor 4011. The reflected
spectra from the fiber grating filter 4035 is then directed back to
the beamsplitter 4025 and onto the detector 4036. In a similar
manner reflections from the fiber grating filters 4037, 4039 and
4041 are directed to the detectors 4043, 4045 and 4047. Note that
the first detector 4036 response includes signals that include
reflections from all the filters 4035, 4037, 4039 and 4041. These
reflections are reduced in intensity through the action of the
beamsplitters 4025, 4051, 4053 and 4055. Since there are n signals
from the n fiber grating spectra reflected by the filters 4035,
4037, 4039 and 4041 that are directed to the output detectors 4036,
4043, 4045 and 4047 a system of equations is established that can
be used to separate the signals for each individual sensor 4011,
4013, 4015 and 4017. The reference detector 4029 can be used to
establish a baseline to compensate for light source 4001 and system
level fluctuations before the string of fiber grating filters 4035,
4037, 4039 and 4041.
[0129] A second means to operate the system of FIG. 53 is to have
the light source 4001 be tunable over the range of the fiber
grating sensors 4011, 4013, 4015 and 4017. In this case as the
light source is tuned over fiber grating 4011 a reflection off this
grating reflects off the filter 4035. A portion of the reflected
signal is directed to the output detector 4029 that can be
referenced against the output monitoring detector 4037. In a
similar manner fiber grating sensor 4013 can be monitored via fiber
grating filter 4037 using the output detector 4043. Fiber grating
sensor 4015 can be monitored via fiber grating filter 4039 and
output detector 4045. Fiber grating sensor 4017 can be monitored
via fiber grating filter 4041 and output detector 4047. Since only
one fiber grating is illuminated at a time the signals on the
output detectors 4036, 4043, 4045 and 4047 are not mixed and it is
not necessary to solve a series of equations. The limitations of
this approach rather than the first one described in association
with FIG. 53 involve the speed with which the light source may be
tuned limiting the overall response of the system and the cost of
the tunable light source relative to a broadband one such as a
light emitting diode.
[0130] FIG. 54 is similar to FIG. 53 with the addition of the
reference detectors 4061, 4063 and 4065 to aid in eliminating
errors due to component induced intensity fluctuations in the
system.
[0131] FIG. 55 shows a system that also is a single fiber output
configuration. In this case the light source 4301 may be a
broadband light source or a tunable laser diode. When the light
source is a broadband light source that illuminates a series of
fiber grating sensors 4303 . . . 4305 simultaneously, the light
reflected off the fiber gratings 4303 and 4305 is split by the
coupler 4307 into the light beam 4309. A tap coupler 4311 is used
to couple a small amount of light to the reference detector 4313
that monitors system level light fluctuations. A combination fiber
grating filter/beamsplitter 4315 is used to modulate light
reflected from the fiber grating sensor 4303 onto the output
detector 4317. A combination fiber grating filter/beamsplitter 4319
is used modulate light reflected from the fiber grating sensor 4305
onto the output detector 4321. By taking the ratio of the outputs
of detectors 4317 and 4313 the spectral fluctuations of fiber
grating sensor 4303, which is centered about .lambda..sub.1, can be
tracked and environmental changes measured. Similarly by taking the
ratio of the outputs of detectors 4321 and 4313 the spectral
fluctuations of the fiber grating sensor 4305 which is centered
about .lambda..sub.n can be tracked and environmental changes
measured.
[0132] Many changes, modifications, alterations and other uses and
applications which do not depart from the spirit and scope of the
invention are deemed to be covered by the invention which is
limited only by the claims which follow.
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